This invention concerns a method of outside deposition by substantially hydroxyl ion-free plasma, onto a silica rod, possibly doped to modify its refractive index, by reacting a silicon compound, such as SiCl.sub.4 for example, as well as any desired dopants, with oxygen in the presence of a plasma created by induction with the help of a high frequency generator. It also concerns a device for implementing the method.
The inventive method and device serve in particular to fabricate preforms intended to be subsequently transformed by drawing into optical fibers for telecommunications cables It is known that the linear attenuation coefficient of such optical fibers is closely related to the hydroxyl ion content of the silica from which they are made, such ions causing a certain absorption of the radiation in the wavelength ranges of the radiation habitually transmitted by the fibers, and at 1.4 micron in particular.